Antennas are used for detecting electromagnetic radiation (EMR) of a particular frequency.
As is well known, frequency (f) of a wave has an inverse relationship to wavelength (generally denoted λ). The wavelength is equal to the speed of the wave type divided by the frequency of the wave. When dealing with electromagnetic radiation (EMR) in a vacuum, this speed is the speed of light c in a vacuum. The relationship between the wavelength λ of an electromagnetic wave its frequency f is given by the equation:
  f  =      c    λ  
As shown in FIG. 1, a typical antenna 10 is formed to detect electromagnetic waves having a certain frequency f, with a corresponding wavelength (λm). This desired frequency may be referred to herein as the desired detection frequency. The antenna 10 is a so-called quarter wavelength antenna, and its length is a multiple (preferably an odd multiple) of a quarter of the desired detection wavelength, i.e., an odd multiple of ¼ λm.
Note that when a electromagnetic wave (W) with wavelength λm is incident on the antenna 10, this causes a standing wave (denoted by the dashed line in the drawing) to be formed in the antenna. The standing wave is reflected of the end of the antenna, to form a second standing wave (denoted by the dotted line in the drawing). The wavelength of the standing wave is ½ λm.
When an electromagnetic wave travels through a dielectric, the velocity of the wave will be reduced and it will effectively behave as if it had a shorter wavelength. Generally, when an electromagnetic wave enters a medium, its wavelength is reduced (by a factor equal to the refractive index n of the medium) but the frequency of the wave is unchanged. The wavelength of the wave in the medium, λ′ is given by:
      λ    ′    =            λ      0        n  where λ0 is the vacuum wavelength of the wave. Note that the antenna 10 shown in FIG. 1 is formed of an homogenous material, typically a metal.
It is desirable to have more selectivity/sensitivity to specific frequencies in antenna detectors.